The present invention relates generally to cold deforming of metal and more particularly to a method for expanding an elongated tubular member in directions transverse to the longitudinal axis of the member.
Expansion of tubular members for work hardening, sizing and rounding is old and well known. Conventional methods for expanding tubular members do so by applying forces to the interior wall surfaces of the member.
For example, one such method old in the art employs a mechanical pipe expander. The expander includes an expander head comprising a plurality of circumferentially spaced radially expandable die segments that are disposed around a cone located near the end of a drawbar. The pipe is expanded by advancing the pipe over the expander head and then forcing the drawbar cone against the dies to cause the dies to move radially outwardly against the interior wall surface of the member.
Methods employing the above-described mechanical pipe expander suffer from a number of limitations and disadvantages, however. Tubular members of a length greater than the length of the expander head dies must be expanded incrementally, which is time consuming and expensive. Members having a large thickness to inside diameter ratio may not be expandable at all by this method where the expander head and drawbar cannot be made small enough to fit inside the pipe and at the same time provide sufficient force to cause the desired expansion. Application of the method aggravates any irregularities in the wall thickness of the member, i.e. the thinner areas get thinner. Because the expansion is performed incrementally, the member will become less straight due to differential strains impressed upon the member adjacent to the zone of expansion. When the expander head is retracted, the member will spring back to some degree, which must be accounted for. Little control can be exercized over non-uniformities in the outer diameter of the member. And finally, the expansion force cannot exceed the true ultimate strength of the member, and thus the true ultimate strength of the member cannot be increased.
Another conventional expansion method employs fluid pressure inside of the tubular member. A liquid, for example, may be pumped into the sealed interior of a member until the pressure of the liquid is sufficient to cause the member to expand. Alternatively, explosives may be located inside of the member to create a percussive force sufficient to cause an expansion.
Methods employing the above-described hydrostatic expansion suffer from many of the disadvantages described in regard to the mechanical pipe expander. Application of the method aggravates any irregularities in the wall thickness of the member. When the expansion pressure is relieved, the member will spring back to some degree, which must be accounted for. It is difficult to control the amount of expansion with the hydrostatic method. The expansion force cannot exceed the true ultimate strength of the member, and thus the true ultimate strength of the member cannot be increased. In addition, the hydrostatic expansion method requires a time-consuming sealing of the member as well as expensive pumping equipment or explosive devices. And, enormous pressures may be required to expand members having thick walls, exacerbating the pumping and sealing requirements.
Expansion of one tubular member inside of another member having an axially extending tubular cavity for purposes of lining the other member with the expanded member is also old in the art. The conventional methods described above can be employed for this purpose by inserting the liner member inside of the other member and expanding the two together. The materials comprising the liner member and the other member are chosen such that the other member will spring back more than the liner member after expansion, thus capturing the liner member. These methods are disadvantageous, however. The choice of materials is limited by the relative spring back requirements, and the expansion force required must be sufficient to expand both the liner member and the other member at the same time.